Interaction of Vitamin K1 and Vitamin K2 with Dimyristoylphosphatidylcholine and Their Location in the Membrane

Vitamin K₁ and vitamin K₂ play very important biological roles as members of chains of electron transport as antioxidants in membranes and as cofactors for the posttranslational modification of proteins that participate in a number of physiological functions such as coagulation. The interaction of these vitamins with dimyristoylphosphatidylcholine (DMPC) model membranes has been studied by using a biophysical approach. It was observed by using differential scanning calorimetry that both vitamins have a very limited miscibility with DMPC and they form domains rich in the vitamins at high concentrations. Experiments using X-ray diffraction also showed the formation of different phases as a consequence of the inclusion of either vitamin K at temperatures below the phase transition. However, in the fluid state, a homogeneous phase was detected, and a decrease in the thickness of the membrane was accompanied by an increase in the water layer thickness. ²H NMR spectroscopy showed that both vitamins K induced a decrease in the onset of the phase transition, which was bigger for vitamin K₁, and both vitamins decreased the order of the membrane as seen through the first moment (M₁). ¹H NOESY MAS-NMR showed that protons located at the rings or at the beginning of the lateral chain of both vitamins K interacted with a clear preference with protons located in the polar part of DMPC. On the other hand, protons located on the lateral chain have a nearer proximity with the methyl end of the myristoyl chains of DMPC. In agreement with the ²H NMR, ATR-FTIR (attenuated total reflectance Fourier transform infrared spectroscopy) indicated that both vitamins decreased the order parameters of DMPC. It was additionally deduced that the lateral chains of both vitamins were oriented almost in parallel to the myristoyl chains of the phospholipid.

Phosphatidylinositol-4,5-bisphosphate enhances anionic lipid demixing by the C2 domain of PKCα

The C2 domain of PKCα (C2α) induces fluorescence self-quenching of NBD-PS in the presence of Ca²⁺, which is interpreted as the demixing of phosphatidylserine from a mixture of this phospholipid with phosphatidylcholine. Self-quenching of NBD-PS was considerably increased when phosphatidylinositol-4,5-bisphosphate (PIP₂) was present in the membrane. When PIP₂ was the labeled phospholipid, in the form of TopFluor-PIP₂, fluorescence self-quenching induced by the C2 domain was also observed, but this was dependent on the presence of phosphatidylserine. An independent indication of the phospholipid demixing effect given by the C2α domain was obtained by using ²H-NMR, since a shift of the transition temperature of deuterated phosphatidylcholine was observed as a consequence of the addition of the C2α domain, but only in the presence of PIP₂. The demixing induced by the C2α domain may have a physiological significance since it means that the binding of PKCα to membranes is accompanied by the formation of domains enriched in activating lipids, like phosphatidylserine and PIP₂. The formation of these domains may enhance the activation of the enzyme when it binds to membranes containing phosphatidylserine and PIP₂.

Curcumin disorders 1,2-dipalmitoyl-sn-glycero-3-phosphocholine membranes and favors the formation of nonlamellar structures by 1,2-dielaidoyl-sn-glycero-3-phosphoethanolamine

Curcumin is a polyphenol present in turmeric, a spice widely used in Asian traditional medicine and cooking. It has many and diverse biological effects and is incorporated in cell membranes. This paper describes the mode in which curcumin modulates the physical properties of 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) and 1,2-dielaidyl-sn-glycero-3-phosphoetnanolamine (DEPE) multilamellar membranes. Curcumin disordered DPPC membranes at temperatures below T(c) as seen by DSC, FT-IR, (2)H NMR, WAXD, and SAXD. The decrease induced by curcumin in T(c) suggested that it is oriented in the bilayer with its main axis parallel to the acyl chains. Above T(c), too, curcumin introduced disorder as seen by infrared spectroscopy which showed that curcumin also alters the conformation of the polar group of DPPC, increasing the percentage of unhydrated C=O groups, but does not form hydrogen bonds with either the C=O group or the phosphate group of DPPC. Small angle X-ray diffraction showed a notable increase in the repeating spacings as a result of the presence of curcumin, suggesting the formation of a rippled phase. Increasing concentrations of curcumin progressively modified the onset and completion of the phase transition and also DeltaH up to a 6:1 DPPC/curcumin molar ratio. A further increase of curcumin concentration did not produce effects on the transition parameters, suggesting that there is a limit for the solubility of curcumin in DPPC. Additionally, when DEPE was used to test the effect of curcumin on the phospholipid polymorphism, it was found that the temperature at which the H(II) phase is formed decreased, indicating that curcumin favors negative curvature of the membrane, which may be important for explaining its effect on membrane dynamics and on membrane proteins or on proteins which may be activated through membrane insertion.

Edelfosine is incorporated into rafts and alters their organization

The effect of edelfosine (1- O-octadecyl-2- O-methyl-rac-glycero-3-phosphocholine or ET-18-OCH3) on model membranes containing 1-palmitoyl-2-oleoyl- sn-glycero-3-phosphocholine/sphingomyelin/cholesterol (POPC/SM/cholesterol) was studied by several physical techniques. The sample POPC/SM (1:1 molar ratio) showed a broad phase transition as seen by DSC, X-ray diffraction, and 2H NMR. The addition of edelfosine to this sample produced isotropic structures at temperatures above the phase transition, as seen by 2H NMR and by 31P NMR. When cholesterol was added to give a POPC/SM/cholesterol (at a molar ratio 1:1:1), no transition was observed by DSC nor X-ray diffraction, and 2H NMR indicated the presence of a liquid ordered phase. The addition of 10 mol % edelfosine increased the thickness of the membrane as seen by X-ray diffraction and led to bigger differences in the values of the molecular order of the membrane detected at high and low temperatures, as detected through the M 1 first spectral moment from 2H NMR. These differences were even greater when 20 mol % edelfosine was added, and a transition was now clearly visible by DSC. In addition, a gel phase was clearly indicated by X-ray diffraction at low temperatures. The same technique pointed to greater membrane thickness in this mixture and to the appearance of a second membrane structure, indicating the formation of two separated phases in the presence of edelfosine. All of these data strongly suggest that edelfosine associating with cholesterol alter the phase status present in a POPC/SM/cholesterol (1:1:1 molar ratio) mixture, which is reputed to be a model of a raft structure. However, cell experiments showed that edelfosine colocalizes in vivo with rafts and that it may reach concentrations higher than 20 mol % of total lipid, indicating that the concentrations used in the biophysical experiments were within what can be expected in a cell membrane. The conclusion is that molecular ways of action of edelfosine in cells may involve the modification of the structure of rafts.

Interaction of the C2 Domain from Protein Kinase Cε with Model Membranes

The C2 domain from protein kinase Cepsilon (PKCepsilon) binds to membranes but does not require Ca2+ to do so. This work examines the mode in which the conformation and organization of the phospholipids present in model membranes are altered by the presence of the C2 domain from PKCepsilon (C2-PKCepsilon). It is concluded from the results of differential scanning calorimetry that the protein shifted the temperature of the gel to the fluid phase transition of pure 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphate (POPA), widening the transition and increasing it to a higher temperature. When POPA was mixed with 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC), the changes in the transition were smaller and no phase separation was observed. Experiments performed using magic angle spinning NMR showed that this C2 domain specifically affected POPA when the phospholipid was mixed with POPC, as indicated by the downfield shift in the isotropic resonance of POPA, the widening of the resonance peak, the decrease in T2, and the decrease in T1 observed at all temperatures. All these effects were quite marked compared with the very small effect observed with POPC, indicating the specificity of the effect. The presence of the C2-PKCepsilon protein changed the conformation of the polar head group of POPA, as shown by infrared spectroscopy. All these results clearly illustrate the electrostatic interaction that takes place between this C2 domain and membranes which contain POPA in the absence of Ca2+.

Effects of the anti-neoplastic agent ET-18-OCH3 and some analogs on the biophysical properties of model membranes

The effect of 1-O-octadecyl-2-O-methyl-sn-glycero-3-phosphocholine (ET-18-OCH(3), edelfosine), and six other analog asymmetric phosholipids on the physical properties of 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC) model membranes was studied using differential scanning calorimetry (DSC), (31)P-nuclear magnetic resonance ((31)P NMR) and X-ray diffraction. DSC data revealed that, at concentrations of 40mol% and higher, a new type of mixtures with higher T(c) and narrower transitions appeared with all the asymmetric lipids studied. At very high concentrations of these lipids (50-80 mol%), destabilization was observed in the systems probably because of the formation of micelles or small vesicles. In all cases, the asymmetric lipids at concentrations of 40 mol% induced the formation of interdigitated structures in the lamellar gel phase, as deduced from X-ray diffraction. The asymmetric phospholipids were also added to 1,2-dielaidoyl-sn-glycero-3-phosphoethanolamine (DEPE) model membranes and DSC data revealed that the lipids primarily affected transition from the lamellar gel (L(beta)) to the lamellar liquid crystalline (L(alpha)) phase in two aspects: the transition temperature was reduced, and the transition itself became broader and smaller. The lamellar liquid crystalline (L(alpha)) to inverted hexagonal phase (H(II)) transition was also affected, as detected by DSC and (31)P NMR data. Increasing concentrations of the asymmetric lipids reduced the formation of inverted hexagonal phases, which were completely inhibited in the case of ET-18-OCH(3). Since these compounds have been shown to have important biological actions through the plasma membrane, these results may help to understand the mechanism of action of these compounds. In addition these asymmetric lipids were tested for their capacity to induce cell apoptosis, and only ET-18-OCH(3) was found to have a clear effect, thus suggesting that the apoptotic effect is not exerted through changes in the biophysical properties of model membranes.

Modulation of the Membrane Orientation and Secondary Structure of the C-Terminal Domains of Bak and Bcl-2 by Lipids

Infrared spectroscopy was used to study the secondary structure of peptides which imitate the amino acid sequences of the C-terminal domains of the pro-apoptotic protein Bak (Bak-C) and the anti-apoptotic protein Bcl-2 (Bcl-2-C) when incorporated into different lipid vesicles. Whereas beta-pleated sheet was the predominant type of secondary structure of Bak-C in the absence of membranes, the same peptide adopted different structures depending on lipid composition when incorporated into membranes, with the predominance of the alpha-helical structure in the case of DMPC and other phospholipids, such as POPC and POPG. However, beta-pleated sheet was the predominant structure in other membranes containing phospholipids with longer fatty acyl chains and cholesterol, as well as in a mixture which imitates the composition of the outer mitochondrial membrane (OMM). Similarly, Bcl-2-C adopted a structure with a predominance of intermolecularly bound pleated beta-sheet in the absence of membranes, with alpha-helix as the main component in the presence of DMPC and POPG, but intermolecular beta-sheet in the presence of EYPC and cholesterol. Using ATR-IR, it was found that the orientation of the alpha-helical components of both domains was nearly perpendicular to the plane of the membrane in the presence of DMPC membranes, but not in EYPC or OMM membranes. (2)H NMR spectroscopy of DMPC-d(54) confirmed the transmembrane disposition of the domains, revealing that they broadened the phase transition temperature, although the order parameter of the C-D bonds was not affected, as might have been expected for intrinsic peptides. When all these results are taken together, it was concluded that the domains only form transmembrane helices in membranes of reduced thickness and that hydrophobic mismatching occurs in thicker membranes, as happens in the membrane imitating the composition of the OMM, where the peptides were partially located outside the membranes.